24 ^« Introduction to Zoology. 



preceding terms, and leaves only a necessity for the addition of a few 

 points respecting the particulars in which the animal differs from those of 

 his variety. 



The distinction of species then, as the term is generally at present 

 adopted, rests upon modifications of organs of the least importance to life ; 

 but still it is upon the accurate determination of these differences that the 

 whole superstructure of order and class depends, and by attention to them 

 and careful comparison of them with peculiarities of a similar character in 

 other animals, that species are erected into genera, genera into orders, 

 orders into classes, and so on ; the difference between each abstraction in- 

 creasing according to its distance from the original group, being at its 

 minimum between one species and another, and at its maximum between 

 the two principal divisions of animals, the vertebrated and the inverte- 

 brated. 



When the relative importance of each system of organisation, composing 

 the animal series, and the value of the organs of each system with reference 

 to each other, is thus determined, it becomes easy to assign to any given 

 animal or organ the place it should occupy in the great animal scale j and 

 since a certain amount of difference is alone necessary to constitute a dis- 

 tinctive grcupe, it is obvious that neither species nor genus are at all 

 dependent upon the number of animals composing them, the existence of a 

 single animal being sufficient to form either the one or the other, without 

 reference to its size or importance. 



Before we proceed to consider somewhat at length the conditions of all, 

 and more particularly of animal life, we shall, to make our sketch less in- 

 complete, say a few words respecting the material upon the organisation of 

 which those conditions depend ; in fact, to shew how the tools are made, 

 before we consider the work which they have to execute. 



Tlie original or ultimate elements of which organic animal matter is 

 composed, and into which it may by destructive distillation be resolved, 

 are carbon, oxygen, hydrogen, and nitrogen, besides certain other unde- 

 compounded substances, which are too irregular in their appearance, or 

 too minute in their quantity, to deserve a place in a sketch like the present. 



A milder method of analysis gives us those elements in their first and 

 simplest combinatiors, as gelatine, albumen, and fibrine, and some others 

 of less importance, the materials out of which the organised body is imme- 

 diately formed, and which are termed accordingly its proximate elements. 



All those are included under the term of chemical organic constituents ; 

 and their combinations differ not only in chemical composition, but in 

 mechanical structure ; and being separable by the scalpel of the anatomist 

 or other mechanical force, are called the anatomical elements. 



Anatomists have long sought after the ultimate anatomical element into 

 which all structure might be resolved, but their search has not been very 

 successful. This element is generally considered to be a fibre, of the size 



